The present invention relates generally to the testing of used lubricating oil, such as lubricating oil in an internal combustion engine and, more particularly, to optical methods and apparatus for evaluating the condition of lubricating oil to determine whether the oil should be changed.
It is well known that the life expectancies of internal combustion engines are heavily influenced by the rate of wear of lubricated surfaces. When good lubricating oil quality is maintained, small automotive engines may exceed 5,000 hours (equivalent to about 150,000 miles) of operation with virtually no wear. At the other extreme, engines run with badly contaminated oil wear rapidly and may fail before even a small fraction of the potential engine life is realized.
Although most engine users realize that engine life is directly tied to maintaining good oil quality, there is much less certainty concerning what is good oil quality and how good oil quality should be maintained. Most engine manufacturers recommend fixed oil change schedules based on elapsed time, elapsed vehicle miles, and/or elapsed hours of oil use. However, it is well known that oil life is a complex function of operating conditions, weather, engine condition, and time-in-use, as opposed to a fixed mileage or time.
Thus, many sophisticated large engine operators such as the military, railroads and some fleet operators rely on periodic in-service oil analysis rather than fixed oil change intervals so that oil is changed only when it is degraded. The key analysis method used for reciprocating engines is atomic spectrometry. This technique involves atomizing (burning) the oil sample and then measuring either absorption or emission of specific wavelengths of ultraviolet radiation associated with elements which may be present in the oil as a result of wear. The technique provides information about both the quantity and species of debris elements present in the oil. Results are usually reported in parts per million of wear metals by weight. When oil-wetted engine metals are reported in the used oil which were not present in the original oil, it is assumed that they are the result of wear. A full oil analysis not only provides information about the quality of the oil, but also gives the operator insight into the condition of the engine from which the oil sample was taken.
Unfortunately, laboratory oil analysis is expensive and may not always be cost-effective for small engine users. Accordingly, a variety of lower-cost approaches have been proposed, many optically based. Simple measurement of light attenuation by an oil sample is disclosed, for example, in Schoenberg U.S. Pat. No. 1,940,772, Borg U.S. Pat. No. 2,889,736 and Yamano U.S. Pat. No. 4,003,661. A more sophisticated system, which is said to measure wear debris contamination independently of changes in color or opacity, is disclosed in Merritt et al U.S. Pat. No. 3,892,485 and employs two separate detectors for respectively measuring attenuation and scattering of light from a source applied to a volume of oil. The attenuation measurement is employed in a feedback loop to vary the intensity of the light source as compensation for changes in opacity. Another system, disclosed in Skala U.S. Pat. No. 3,790,279, similarly employs separate light scattering and light attenuation measurements. A calculation based on these separate measurements is said to provide an indication of contamination.
Other optically-based approaches have been proposed. Miller et al U.S. Pat. No. 3,049,964 discloses a method for detecting impurities wherein a drop of oil is applied to filter paper. Solid material remains in the center as a dark spot, while oil spreads. A light bulb illuminates the filter paper and spot, and measurement of reflected light indicates the size of the dark spot of impurities. A technique known as "Ferrography" is disclosed in Jones et al U.S. Pat. No. 4,492,461.
Nevertheless, there remains a need for a low-cost technique and instrument for accurately detecting metallic wear debris in used oil, such as used motor oil, particularly in the presence of high levels of carbon particulate.
By way of further background, discussed below in detail are characteristics of lubricating oil, a number of specific wear mechanisms, the significance of wear rate being nonlinear with respect to time, and reasons why metallic wear debris concentration is a good indicator of a decrease in oil quality.
Oil is used to prevent direct contact between opposing metal surfaces which are in motion relative to one another. In essence, the normal load between the surfaces is supported by the liquid, thereby minimizing shear forces. In addition, modern motor oils contain anti-wear additives which modify the metal surfaces, resulting in lower levels of friction and wear than could be provided by the base oil stock alone. Significant wear normally occurs only when the oil and/or additives are allowed to degrade.
The ability of a lubricating oil to prevent wear is impaired by the presence of contaminates. Solid particles in the oil cause wear by simple mechanical contact with oil-wetted surfaces, by increasing effective oil viscosity, and by disrupting hydrodynamic films. In the case of very small (sub-micron-sized) particles, wear results from mild abrasive (erosive) action. However, when a particle reaches the oil thickness dimension between opposing, moving surfaces, catastrophic wear results from galling and disruption of oil films. Large metal particles are gouged providing a higher level of large particle contamination. Once a critical large particle population is reached, this cycle becomes self-accelerating and wear rates increase exponentially until the oil is changed or the machine fails.
Liquid contaminates are also of concern. Water exposed to oxides of sulfur and nitrogen formed in combustion processes forms acids. Oxidation of the oil results in viscosity increases accompanied by reduced flow rates and decreased convective heat transfer. Eventually, tars and lacquers form, further blocking heat transfer, blocking filters, and restricting oil flow in narrow passages.
In reciprocating, internal-combustion engines, oil degradation rates are relatively high due to the exposure of the oil to the combustion process. A fundamental cause of oil contamination in engines is the buildup of small, elemental, carbon particulates in the oil. In low concentrations these particles are relatively harmless. However, once concentration levels are sufficient to allow agglomeration, the particles bind together. The rate with which the particles are formed by combustion is primarily a function of the carbon fraction of the fuel, with very high deposition rates in diesels and much lower rates in natural-gas-fueled engines. Other fuel-dependent contaminates include sulfur, lead and water. Other causes of lubricating oil degradation which are exacerbated in reciprocating engines include exposure to condensation, fuel dilution, and the existence of high oil shear rates. As noted above, the engine-dependent factors which promote oil degradation are not constants and vary with engine service environment, engine condition, and engine design.
There are a number of specific wear mechanisms which apply to reciprocating engines, as discussed in the next several paragraphs.
Adhesive wear occurs when direct, "dry" contact is allowed between the opposing metal surfaces. Such contact results in severe friction heating and ultimately in adhesion (seizure) of opposing surfaces. This type of wear typically results from an interruption of the oil supply such as when a hot engine is shut-off: the hot, thin oil drains from lubricated surfaces; it cools overnight, becoming viscous; and finally, the engine is started with only residual lubrication from the remaining oil and anti-wear coatings. The amount of damage done is determined by a race between the rate of removal of the anti-wear coating and the arrival of the thick, slow-moving oil. Obviously, damage will be severe if the anti-wear additives have been exhausted or the low-temperature oil viscosity has increased.
A secondary effect of adhesion is to contaminate the oil with work-hardened wear debris and to roughen the metal surfaces, causing abrasive wear not only at starting but for hours afterward.
Abrasive wear, mentioned above, occurs when the thickness of the oil film which separates moving surfaces is exceeded by the dimensions of solid particles suspended in the oil or the height of surface protuberances (asperities) on the opposing surfaces. This wear mechanism is similar to the familiar action of abrasive papers or grinding wheels, material removed from one or more of the opposing surfaces by localized, intermittent contact. Abrasive wear rates increase with decreasing oil viscosity or increasing particle size or concentration. Lower oil viscosities lead to thinner lubricating films, while large, coarse particles remove material more rapidly than small, fine particles.
Erosive wear occurs when particle-laden oil impinges on a surface. Material is removed from the surface by the scouring action. As is the case with abrasive wear, a key factor in reducing erosive wear is maintaining a clean, particle-free oil.
Surface fatigue results from cyclic normal loading (vibration) of the metal surfaces rather than sliding friction. Cam-follower surfaces are subject to this type of wear which results in pitting of the surfaces. Again, particle contamination provides a source of higher point loading, aggravating wear.
Corrosive wear is the loss of material as a result of chemical modification of the oil-wetted surfaces. Acids form in engine lubricating oils as a result of exposure to combustion byproducts and condensate moisture. In general, the presence of acids in a lubricating oil will weaken metal surfaces, making them much more vulnerable to mechanical wear. Motor oils contain sacrificial additives to combat corrosion. Once they are consumed, the rate of corrosive wear escalates.
In addition, both the base oil and the anti-wear additives are susceptible to changes in molecular structure at high temperatures. In the case of the base oil, oxidation results in viscosity increases, particulate formation, and solid deposits known as lacquer. Motor oils contain sacrificial additives to control oxidation. However, oxidation rates may also be increased by the catalytic action of metallic wear debris accumulating in the oil.
A key point is that most of the wear processes are not linear with respect to time, and it is correspondingly significant that oil quality does not degrade in uniform fashion across engine types or even in a single engine as it ages. Typically, wear remains at very low levels during most of the oil's useful life, and then drastically increases as one or more of the additives are depleted. This nonlinear wear rate is one of the primary reasons for being very conservative in selecting an oil change interval; if one misjudges, a great deal of wear will take place in a relatively short interval. Most engine manufacturers suggest that engine oil be changed on a periodic basis dictated by the elapsed engine operating hours or vehicle miles since the last oil change. However, in view of the wide variation in oil degradation rates, this is questionable practice.
By the same token, it is folly to attempt to maintain oil quality on the basis of infrequently conducted tests. Tests must be conducted more frequently as the oil ages, and results should be trended to detect the onset of the increase in wear.
The best measure of the lubricating quality of an oil is the absence or presence of wear. By definition, if wear is not occurring, then the machine is well-lubricated. Thus, wear debris concentration is an objective test of the primary oil function, i.e., prevention of wear. If wear debris is not present in the oil after a period of residence in the engine, then it is axiomatic that the oil is providing adequate lubrication and need not be changed. The presence of wear can be detected by examining the used engine oil for the presence of wear debris. If the oil is of good quality and not degraded, wear debris levels will remain very low. If, on the other hand, the oil is badly degraded, wear debris levels will increase, as will the mean particle size of the debris.
Illustrating these points, FIGS. 1 and 2 are plots of total wear debris levels in randomly sampled engine oils as a function of engine operating hours and engine miles, respectively, since the last oil change. From these plots, it can be seen that there is little or no relationship between oil service time and oil lubricating quality, demonstrating the problems with oil change intervals based on service time measures. It may be noted that the data shown in FIG. 1 is based on oil samples from engines fueled with low-carbon-fraction natural gas, which is primarily methane (1:4 carbon/hydrogen ratio). The oil samples on which FIG. 1 is based are various SAE 10W-30, 30 and 40 motor oils. FIG. 2 is based on oil samples from engines fueled with gasoline (7:16 carbon/hydrogen ratio). In FIG. 2, the "x" sample points are for SAE 20W-50 oil, and the "o" sample points are from SAE 10W-30 oil.
Most small machine and automotive oil maintenance programs are based on periodic oil changes. However, from the foregoing it will be appreciated that oil changes based on fixed service intervals are a questionable maintenance practice. The optimal oil maintenance program is one in which oil changes are triggered by the results of an evaluation of oil condition, not the passage of an arbitrary service period.